How to read 'Surface' weather maps

Weather maps come in myriads of styles with each providing different levels of information. However, there are some common features typically found of these images.

In the section about the Origin of Wind we have already seen the source of the "highs" and "lows". But how are the boundaries between air masses depicted? We draws lines, called "fronts".

Fronts are usually detectable at the surface in a number of ways. Winds often "converge" or come together at the fronts. Also, temperature differences can be quite noticeable from one side of a front to the other side. Finally, the pressure on either side of a front can vary significantly.

Fronts

Cold Front

Cold fronts are depicted by blue line with triangles pointing in the direction of motion. Cold fronts demarcate the leading edge of a cold air mass displacing a warmer air mass.

Phrases like "ahead of the front"and "behind of the front" refer to its motion. So being "ahead of the cold front" is being in the "warm" air mass and "behind of the cold front" is in the cold air mass.

Also remember however, the terms "cold" and "warm" are relative. So, it is still called a cold front even in summer if the temperature only lowers from, for example, 95°F (35°C) ahead of the front to near 90°F (32°C) behind the front.

Cold fronts nearly always extend anywhere from a south direction to a west direction from the center of low pressure areas and never from the center of high pressure systems.

Warm Front

A warm front is the leading edge of a relatively warmer air mass replacing a colder air mass. A warm front are depicted by a red line with half-moons located on the side of the direction of its motion.

Like cold front, warm fronts also extend from the center of low pressure areas but on nearly always on the east side of the low.

Here is an example of a location that experiences typical warm frontal passage followed by a cold frontal passage: Clouds lower and thicken as the warm front approaches with several hours of light to moderate rain. Temperatures are in the 50s with winds from the east.

As the warm front passes, the rain ends, skies become partly cloudy and temperatures warm into the mid 70s. Winds become gusty from the south. A few hours later, a line of thunderstorms sweeps across the area just ahead of the cold front. After the rain ends and the front passes, winds shift to the northwest and temperatures fall into the 40s and skies clear.

Stationary Front

If the front is essentially not moving (i.e. the two air masses on either side are not moving perpendicular to the front) it is called a stationary front. Stationary front are depicted by an alternating red and blue line with a triangle on the blue portion and half moon on the opposite side of the red portion of the line.

A cold front (or warm front) that stops moving becomes a stationary front. The difference in temperature and wind direction from one side of a stationary front to the other is generally not large but there can be times where the difference is stark.

Occluded Front

The cold air mass is moving faster than the cool air mass. As the two fronts converge the cold air undercuts the cooler air mass.

Cold fronts typically move faster than warm fronts, so in time they can "catch up" to warm fronts. As they do the warm air mass is forced up forming an occlusion.

The surface location of the occluded front is directly below the convergence point of the warm, cool and cold air masses. Occluded fronts points to a decrease in intensity of the parent weather system and are indicated by a purple line with alternating triangles and half-moons on the side of its motion.

While there is no difference in how they are depicted on a weather map, there are two types of occlusions; cold and warm.

The cool air mass is out running the cold air mass. But the because the cold air mass is more dense, the cool (less dense) air is forced up.

Cold occlusions are the most common where the cold front over takes the warm front and also undercuts the cooler air mass ahead of the warm front.

Warm occlusions occur when the air associated with the "cold" front is actually not a cold as the air mass associated with the warm front. The warm air is forced up as before but the colder, more dense, air mass ahead of the warm front remains at the surface forcing the air mass associated with the cold front up as well.

Other Boundaries

Dry Line

Dry air, being more dense undercuts the light moist air forcing it up.

A dryline marks the boundary between a moist air mass and dry air mass. It typically lies north-south across the central and southern high Plains states during the spring and early summer, where it separates moist air from the Gulf of Mexico (to the east) and dry desert air from the southwestern states (to the west).

The dry line typically advances eastward during the afternoon and retreats westward at night. However, a strong storm system can sweep the dry line eastward into the Mississippi Valley, or even further east, regardless of the time of day.

A typical dry line passage results in a sharp drop in humidity, a rise in temperatures, clearing skies, and a wind shift from south or southeasterly to west or southwesterly. (Blowing dust and rising temperatures also may follow, especially if the dry line passes during the daytime.) These changes occur in reverse order when the dry line retreats westward.

Since drier air is more dense than moist air, as the dryline moves east it forces moist air up into the atmosphere. Therefore, severe and sometimes tornadic thunderstorms can develop along a dry line or in the moist air just to the east of it.

Squall Line

This is a line of thunderstorms that generally form along a front but the storms move ahead of the front. As the rain cooled air under the thunderstorms begins to surge forward new thunderstorms form on the leading edge of the outflow.

The outflow acts like a cold front with an increase of forward speed and therefore an increase in forward speed of the line of thunderstorms. Squall lines are most notably seen in derechos.

Other Symbols

Trough

A trough is not a boundary but an elongated area of lower air pressure. There are changes in wind direction across a trough but there is no change in air mass.

While not specificity a surface boundary, troughs reflect the change in atmospheric conditions in the upper atmosphere. As such, troughs can be areas where showers and thunderstorms can form.

Precipitation

Historically, areas of precipitation have been shaded green regardless if it the precipitation is frozen or not. The type of precipitation on weather maps itself also comes in numerous forms. Sometimes the precipitation type is spelled out or, as more often the case, use a wide variety of graphics to indicate type.

Below are some of the more traditional meteorological symbols used on maps to indicate precipitation types.